There is no index.pl in the xf folder. Deleting the domain isn't an option as the domain was created using Plesk and there's to other sites on it. So I can delete and re-add the domain without causing major disturbance.

Yeah I do have ssh access the problem is, I don't want to delete the whole domain. Because that removes the database, the files and so on. I would have to then re-upload everything and one of the sites on that domain is a big-ish forum. Downtime isn't an option.
So I guess I'll just use another domain and play around with that.

I get the error when I try to add a new language in the ACP. Well the error I get is "Server Error" and nothing else. You can see it in the video.
The error message is from the error log file.

As for changing stuff in apache - I don't think I should do that when Plesk is running the Apache server. I don't want to mess that up.
I've just recently bought a new iMac which I'll install MAMP on. So I'll install XF on that and then I'll be able to do whatever I want on it. I'll post again here then.